The present application is based on patent application No. 2002002-194760 filed in Japan, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a dispersion compensator for compensating for a deterioration in an optical signal which is caused by a dispersion of a light which is transmitted in an optical transfer element such as an optical fiber which is used for an optical communication. The present invention particularly relates to a dispersion compensator which can compensate a chromatic dispersion and a dispersion slope which are caused in the optical transfer element.
2. Description of Related Art
There has been an increasing requirement for larger capacity in an optical communication system as a traffic in a communication increases according to an Internet which is rapidly used commonly. Conventionally, larger communication capacity has been realized by increasing a transmission speed based on a faster processing speed in an electronic circuit. However, a recent request for increasing the communication traffic exceeds what can be realized by making use of faster processing speed in such an electronic circuit. Presently, it is inevitable to use a wavelength division multiplexing (hereinafter called WDM) method together with the higher speed processing operation by an electronic circuit.
Presently, an optical transmission speed which is commercially realized has a limit such as 10 Gb/s. According to the latest status in this area in the art, a transmission speed such as 40 Gb/s has already been realized under non-commercial condition such as under a Research and Development condition because of an improved electronic device using a composite-semiconductor. Regarding a medium which us used for an optical communication, there is a serious problem wider width in a transmission pulse width caused by a dispersion in that a transmission speed of a light which exists in a transmission medium alters based on a wavelength or a polarization condition of a light in case of a high transmission over 10 Gb/s. It is understood that it is necessary to reduce a decrease an entire system for transmitting a light to zero.
For Methods for decreasing such a dispersion, following methods can be commonly used for example. In a method, dispersion in an optical fiber is reduced by using a dispersion shift fiber which adjust a zero-dispersion wavelength in a fiber to a transmission wavelength. In other method, a dispersion compensation fibers (hereinafter called DCFs) which have an opposite characteristics to an optical fiber such as a single mode fiber (hereinafter called SMF) are disposed in constant intervals have been used commonly.
On the other hand, more strict control for dispersion is required under condition of transmission speed such as 40 Gb/s; thus, it is necessary to correct the dispersion in an optical fiber which is caused by a temperature condition dynamically.
In order to solve such problems, a fiber brag-grating (hereinafter called an FBG) and a virtually-imaged-phased-array (hereinafter called a VIPA) have been proposed. For a dispersion compensator using a VIPA, a dispersion compensator can be named which is disclosed in a patent document such as Published Japanese Translation No. 2000-511655 of PCT. In this dispersion compensator, a light which is ejected from an optical fiber is collimated. After that, the collimated light is condensed and transmitted through a VIPA which is disposed in a focal point of the transmitted light. Thus, a light flux which can be identified according to each wavelength therein is generated so as to be a parallel light. The parallel light is condensed and reflected by a reflection mirror which is disposed in a focal point. The reflected light returns to an optical fiber by reversing the reflected light in the same optical system thereabove.
According to such a dispersion compensator, a light which is outputted from the VIPA is condensed in a different point on the reflecting mirror, and a surface on on the reflection is formed in a certain shape. By doing this, it is possible to generate an optical path length difference. Thus, a chromatic dispersion is compensated because different wavelengths transmit for different distances.
For a member to compensating the dispersion slope, a dispersion compensator which is disclosed in a patent document such as U.S. Pat. No. 6,301,048 is proposed. According to the document, a method is disclosed by which a dispersion and a dispersion slope are compensated concurrently by using a VIPA and a diffracting grating.
An object of the present invention is to provide a dispersion compensator which can compensate a chromatic dispersion and a dispersion slope under condition that an insertion loss can be restricted in a minimum level.
Another object of the present invention is to provide a dispersion compensator and a dispersion compensating system which do not need a large space for realizing functions for varying a dispersion to be compensated and a dispersion slope.
A dispersion compensator according to the present invention is characterized in comprising an angular dispersion element for changing an angle of a light which is ejected from an optical transfer element according to a wavelength in the ejected light, a diffracting optical element having at least a concave reflecting surface so as to diffract the ejected light, and a reflecting mirror having a reflecting surface which is disposed near a focal point in an entire optical system of which surface shape of the reflecting mirror changes in a dispersing direction of the ejected light according the wavelength in the ejected light.
Also, a dispersion compensator according to the present invention is characterized in comprising an angular dispersion element for changing an angle of a light which is ejected from an optical transfer element according to a wavelength in the ejected light, an optical element having a light-condensing function, a diffracting optical element having at least a concave reflecting surface so as to diffract the ejected light, and a reflecting mirror having a reflecting surface which is disposed near a focal point in an entire optical system of which surface shape of the reflecting mirror changes in a dispersing direction of the ejected light according the wavelength in the ejected light.
Also, it may be acceptable that the focal point in which an image is focused by the optical element and the reflecting surface of the reflecting mirror are disposed on a circumference which is formed according to a radius of curvature in the concave reflecting surface in the diffracting optical element.
Also, according to the present invention, a dispersion compensator is characterized in comprising an angular dispersion element for changing an angle of a light which is ejected from an optical transfer element according to a wavelength in the ejected light, an optical element having a light-condensing function, an optical deflecting device for deflecting a light which is ejected from the optical element near a focal point in which the light which is ejected from the optical element is focused, a diffracting optical element having at least a concave reflecting surface so as to diffract the ejected light, and a reflecting mirror having a reflecting surface which is disposed near a focal point in an entire optical system of which surface shape of the reflecting mirror changes in a dispersing direction of the ejected light according the wavelength in the ejected light.
Also, according to the present invention, it may be acceptable that the reflecting surface on the optical deflecting device and the reflecting surface on the reflecting mirror are disposed on a circumference which is formed according to a radius of curvature in the concave reflecting surface in the diffracting optical element.
Also, it may be acceptable that the diffracting optical element is a concave-surfaced diffracting grating, and a grating pitch in the concave-surfaced diffracting grating differs according to an area on a concave-surfaced reflecting surface. Also, it may be acceptable that the grating pitch in the concave-surfaced diffracting grating differs according to an area in a direction orthogonal to a direction of chromatic dispersion by the angular dispersion element which is disposed on the concave reflecting surface.
Also, it may be acceptable that the diffracting grating is provided with a blaze angle.
Also, it may be acceptable that the focal point in which an image is focused by the optical element and the reflecting surface of the reflecting mirror are movable such that the focal point in which an image is focused by the optical element and the reflecting surface of the reflecting mirror maintain such a relationship to be disposed on a circumference which is formed according to a radius of curvature in the concave reflecting surface in the diffracting optical element.
Also, it may be acceptable that the diffracting optical element can be rotated around a fulcrum which is disposed in a center of a curvature in the reflecting surface of the diffracting optical element.
Also, it may be acceptable that the reflecting surface on the reflecting mirror has a power at least in a surface orthogonal to a direction of chromatic dispersion by the angular dispersion element.
Also, it may be acceptable that the reflecting surface on the reflecting mirror has a power at least in a plane which includes a direction of chromatic dispersion by the angular dispersion element.
Also, it may be acceptable that the reflecting surface on the reflecting mirror is formed in rotatively an asymmetrical free-form surface.
Also, it may be acceptable that the reflecting surface on the reflecting mirror is provided with a surface which is disposed diagonally in an incident optical axis at least in a plane which includes a direction of chromatic dispersion by the angular dispersion element.
Also, it may be acceptable that the reflecting surface on the reflecting mirror is movable in a direction approximately orthogonal to an incident optical axis.
Also, it may be acceptable that the reflecting surface on the reflecting mirror is movable at least in a surface orthogonal to a direction of chromatic dispersion by the angular dispersion element.
Also, it may be acceptable that the reflecting surface on the reflecting mirror is movable in a plane which includes a direction of chromatic dispersion by the angular dispersion element.
Also, it may be acceptable that an optical surface or an optical member which has a positive power is disposed in a plane which includes at least a surface in a direction in which a wavelength is dispersed by the angular dispersion element in an optical path between the angular dispersion element and the reflecting surface on the reflecting mirror.
Also, it may be acceptable that the optical surface which has a positive power in a plane which includes a direction of chromatic dispersion by the angular dispersion element is a reflecting surface made by the concave-surfaced diffracting grating.
Also, it may be acceptable that the reflecting surface made by the concave-surfaced diffracting grating is an anamorphotic surface.
Also, it may be acceptable that the reflecting surface made by the concave-surfaced diffracting grating is rotatively an asymmetrical free-form surface.
Also, it may be acceptable that the optical member which has a positive power in a plane which includes a direction of chromatic dispersion by the angular dispersion element is a cylindrical lens which is disposed between the angular dispersion element and the reflecting mirror.
Otherwise, it may be acceptable that the optical member which has a positive power in a plane which includes a direction of chromatic dispersion by the angular dispersion element is an anamorphotic lens or a free-form-surfaced lens which is disposed between the angular dispersion element and the reflecting mirror.
Also, it may be acceptable that the optical element is an optical deflecting device which is provided with a reflecting surface which has a positive power. In the present invention, it may be acceptable that the optical deflecting device is a reflecting diffracting grating.
Also, it may be acceptable that the optical element is a concave-surfaced mirror having a positive power which is disposed between the angular dispersion element and the diffracting optical element.
Also, according to the present invention, it may be acceptable that the concave-surfaced mirror is an anamorphotic concave-surfaced mirror.
Also, it may be acceptable that the optical element is provided with a free-form reflecting surface which is disposed between the angular dispersion element and the diffracting optical element.
In the present invention, an interferometer, a Fabry-Pxc3xa9rot-interferometer, an etalon, a VIPA, a diffracting grating, or a prism can be used for an angular dispersing element.
Also, in the present invention, it may be acceptable that the diffracting grating is provided with a surface having a blaze angle.
According to the present invention, a dispersion compensating system is characterized in comprising a dispersion compensator of the above aspect of the present invention, a signal monitor which monitors a light which is ejected from the dispersion compensator and outputs a signal which contains at least an information for a dispersion of the light or an information for a dispersion slope, and a control device which controls a movement of the reflecting mirror such that at least a dispersion or a dispersion slope is reduced according to the signal which is outputted from the signal monitor.
Also, in a dispersion compensating system, it may be acceptable that a deflecting angle by an optical deflecting device is controlled instead of using a reflecting mirror so as to adjust at least one of the chromatic dispersion or the dispersion slope automatically. Otherwise, it may be acceptable that a position of the diffracting optical element is controlled.